Early detection of breast cancer in women is universally recognized as the most effective deterrent to deaths caused by that disease. Women are therefore encouraged to undergo regular medical examinations including breast X-ray on a routine periodic basis. Because of the large female population requiring such X-ray examinations it is desirable to provide a means for X-raying the breasts of large populatons of women as rapidly and economically as possible and producing the required X-rays in a manner suitable for similarly economical and rapid evaluation.
Apparatus for breast X-ray imaging is currently known. In such apparatus a phosphor intensifying screen and a sheet of film with an emulsion sensitive to the light emitted by the phosphor is encased in a thin bag or envelope of material transparent to X-rays. The envelope is air-tight and is evacuated to bring the film and screen into very close contact so that the X-ray image of the breast formed on the screen may be recorded with high resolution on the film. The envelope containing the screen and film is placed below the X-ray apparatus and the breasts of the subject are placed thereagainst after which time the breasts are exposed to X-radiation which penetrates the breasts and causes an X-ray image containing information as to the nature of the tissue within the breasts to be formed on the phosphor screen which image is then recorded on the sheet of film.
Sheet film sizes of at least 8 .times. 10 inches are generally used and sheet film of this size is relatively expensive. Of substantially greater expense are the intensifying screens employed in the X-ray process. A separate intensifying screen is required for each film package, one film package being required for each exposure. Although the screens are reusable they are quite fragile and often damaged in repeated use, necessitating their frequent replacement. In addition to the expense associated with the use of sheet film and individual screens for each exposure, it is additionally time-consuming and costly to package each sheet of film and screen in a separate envelope, evacuate the envelope and seal it under dust-free and light-free conditions. Furthermore, the time expended in replacing the screen-film envelope after each X-ray exposure limits the number of women who can be accommodated in a mammography mass screening program.
It is known in the art to use smaller areas of film than the conventional 8 .times. 10 inch film. However, in such prior art systems lenses are used to reduce the full-size image of the breast formed on the screen to a smaller image on the film. It is also known in the art to use electronic means to reduce the image formed on the screen, for example on a cathode ray tube, and to record that image on a small area of photographic film. The use of lenses and/or electronic means to conserve film through reduced image size significantly degrades resolution due to aberrations in the lens and loss of information in the electronic processing. Furthermore, and most important, when a lens is used it is only capable of gathering a small fraction of the light photons forming the image which are emitted by the phosphor screen, so that a large fraction of the light photons containing useful information are lost and not utilized in the mammogram.